1. Field of Invention
The present invention applies to the chemical cleaning or combined chemical and mechanical cleaning of heat exchangers or vessels, including nuclear pressurized water reactor (PWR) steam generators. Example materials targeted for removal by cleaning include those that reside on the secondary (boiling) side of heat exchangers or vessels and comprise metallic oxides (e.g., magnetite), metallic species (e.g., copper), other impurities (e.g., mineral species) or waste materials. The method described herein may also be used in conjunction with other deposit or waste management strategies such as dispersants or scale conditioning agent solutions, which are added to the heat exchanger or vessel to mitigate the accumulation of deposits in these systems or to modify the structure of these deposits once accumulation has occurred. The method and system described herein may also be used with decontamination solutions or with other processes for cleaning heat exchangers or vessels, including the removal of waste, such as nuclear waste, from a vessel, heat exchanger or fluid systems where temperature control is required or helpful.
2. Description of Related Art
The removal of deposits from the secondary side of heat exchangers, and more specifically the secondary or boiling side of nuclear pressurized water reactor (PWR) steam generators, has been achieved by both chemical and mechanical means. Chemical means include high and low temperature chemical cleaning, and mechanical means include processes such as pressure-pulse cleaning, water jetting or lancing, or bundle flushes with water or chemical solutions. Chemical means and mechanical means are often combined by performing them concurrently or sequentially.
There are a variety of chemical cleaning processes used to clean heat exchangers and vessels in general, and nuclear steam generators in specific. Many of these processes are described in Frenier, W., “Technology for Chemical Cleaning of Industrial Equipment,” NACE International—The Corrosion Society, 2001. As discussed below, there are two basic types of chemical cleaning processes for power plant heat exchangers and vessels such as PWR steam generators: “on-line” (plant heat) and “off-line” (external heat) cleaning processes. Off-line processes refer to processes in which the supply, heating, pumping, mixing, cooling and draining of the chemical solutions is performed via the installation and use of temporary external equipment. The equipment configurations associated with off-line processes are typically very complex, and require significant time and manpower to set up and operate. However, because the plant is fully shut down during external process applications, this type of process is often considered a preferred method of cleaning for safety, process control and other economic reasons. Off-line processes allow electrochemical corrosion monitoring equipment to be installed inside the vessel such as a steam generator to ensure that no harmful side effects of the cleaning operation are occurring. Liquid samples can also be easily taken via temporary sample lines to monitor the process and to ensure that excessive corrosion of vessel or steam generator internals does not occur during the cleaning process due to off-normal process or chemistry conditions.
Processes that use primary-to-secondary heat transfer to control the temperature of the cleaning process at a power plant such as a PWR are referred to as “plant heat” or “on-line” processes. The equipment setup and manpower requirements are significantly reduced during on-line processes because heating and cooling of the secondary side (locations of deposits) is supplied from the primary side of the plant using plant systems such as decay heat from reactor core (for heating) or the plant residual heat removal (RHR) system (for cooling). As such, no external heating or cooling equipment is required. Because plant heat processes are applied while the plant is “on-line”, there is no access to the vessel such as a steam generator prior to the cleaning. This prevents the installation of corrosion monitoring equipment inside the steam generator. Liquid sampling is also more difficult during “on-line” processes because the vessel such as a steam generator may need to be partially drained back through plant systems in order to obtain a sample of the cleaning solvent. Thus, process monitoring is much more difficult during “on-line” processes. Excessive corrosion and other off-normal chemistry conditions have been known to occur during conventional “on-line” cleaning applications (see “Application of AREVA Inhibitor-Free High Temperature Chemical Cleaning Process against Blockages on SG Tube Supports,” Dijoux, M. et al, presented at “NPC '08 Berlin, International Conference on Water Chemistry of Nuclear Reactor Systems,” held in Berlin, Germany, Sep. 15-18, 2008).
With regard to the cleaning of nuclear steam generators, much of the original research that led to the solvents and processes used today was sponsored by the Steam Generator Owners Group (SGOG) of the Electric Power Research Institute (EPRI) and documented in several reports including EPRI-2976 entitled “Chemical Cleaning Solvent and Process Testing” (April 1983), and EPRI NP-3009 entitled “Steam Generator Chemical Cleaning Process Development” (April 1983).
Other cleaning processes which use less concentrated chemical solvents to partially remove, disrupt or change the characteristics of deposits are described in U.S. Pat. No. 5,841,826 to Rootham et al. (“Rootham I”), U.S. Pat. No. 6,740,168 to Rootham et al. (“Rootham II”), and U.S. Pat. No. 7,344,602 to Varrin et al. (“Varrin”). These processes are typically applied as on-line processes, but may be applied as offline processes based on plant-specific considerations.
In chemical cleaning processes designed for complete removal of deposits, high temperature processes generally refer to those applied, for example, at 285 to 428° F. (140 to 220° C.), see U.S. Pat. No. 5,264,041 to Kuhnke et al. (“Kuhnke”). These processes are usually applied with the temperature maintained by heat transfer from the primary side of the plant, often while the plant is shutting down for maintenance or refueling. As discussed earlier, these processes are referred to as “on-line” processes in the context of chemical cleaning. The primary side of the plant, or reactor coolant system, is the closed loop portion of the PWR plant comprising the fuel, reactor, reactor coolant pumps, the pressurizer, numerous reactor control and safety systems, and the tubes internal to the steam generators. On the other hand, the secondary side is the portion of the plant which includes the outside of the tubes in the steam generators, the steam lines, turbines, condenser, several stages of pumps, and feedwater heaters.
Low temperature processes generally refer to processes applied from, for example, 85 to 285° F. (30° C. to 140° C.), with the temperature maintained by either: (1) primary to secondary side heat transfer (“on-line”), or (2) use of temporary equipment set up outside of the containment building (“off-line”). Temporary equipment typically includes an external heating loop that exchanges heat indirectly with the main chemical cleaning process loop via an external heat exchanger (see discussion below). Heat is typically supplied to the external heating loop by a portable steam boiler, but may also be supplied by electrical heater(s) or by steam from an adjacent power plant. When steam is used, it is condensed on one side of a heat exchanger and not admixed with the cleaning solution (also referred to as indirect heating as opposed to direct steam injection).
In nuclear PWRs, the containment building houses the reactor (primary loop) and the steam generators. Steam produced on the “secondary side” of the steam generators exits the steam generators via steam lines which in turn pass through penetrations in the containment building to supply the turbine-generator. Condensed steam or “feedwater” then returns to the steam generators via separate penetrations in the containment building from the condenser through the auxiliary building which houses the aforementioned feedwater heaters, pumps and other equipment. Temporary penetrations at the containment building boundary are also available but generally limited in size and number. These penetrations are often used to connect temporary equipment to the steam generators, but the limited number and size of the penetrations makes it difficult to link or interconnect complex cleaning equipment configurations located outside of containment to the steam generators.
At PWRs, there are two basic types of steam generators (SGs). One type is known as a recirculating steam generator (RSG). In an RSG, the tubes which constitute the primary to secondary side boundary are vertically oriented and U-shaped, such that the primary coolant enters and exits the SG near the bottom. The tube “bundle” can consist of thousands of tubes. The other type of steam generator is known as a once-through steam generator (OTSG). In an OTSG, the tubes are straight and vertically oriented such the primary coolant enters at the top of the SG and exits at the bottom. In both RSGs and OTSGs, steam is produced outside the tubes. Both types of steam generators may require periodic chemical cleaning or conditioning to reduce concerns with thermal efficiency and corrosion of the tube materials.
In general, a large amount of equipment is required for off-line nuclear steam generator chemical cleaning processes that use temporary equipment for preparing, heating, cooling and recirculating chemical cleaning solvents. Requirements for the temporary cleaning equipment is well-described in Partridge, M. J. and J. A. Gorman, “Guidelines for Design of PWR Steam Generator Chemical Cleaning Systems,” Electric Power Research Institute, Palo Alto, Calif., January 1983. This reference describes the methods employed for off-line “external heat” chemical cleaning of PWR steam generators using either specially designed flow loops or by a process known as “fill, soak and drain” (also described in U.S. Pat. No. 5,257,296 to Buford et al. (“Buford”)) in which chemical solvents are mixed, preheated and pumped into the steam generator, allowed to soak until the temperature drops to an unacceptable level, followed by draining and reheating of the solvent external to the steam generators, and then finally re-injecting the re-heated solvent back into the steam generator. This process may be repeated multiple times until the steam generators are considered clean, at the expense of increasing overall cleaning time.
Partridge and Gorman describe the use of steam for indirectly heating of solvents (in an “external heat” process) by passing steam through a heat exchanger integral to the temporary chemical cleaning equipment system located outside of the containment building. In this configuration, steam is available from a portable boiler, but may also be supplied from an adjacent power plant.
U.S. Pat. No. 7,302,917 to Remark et al. (“Remark”) discloses an on-line plant heat steam generator chemical cleaning process that involves introducing a chemical cleaning solvent to the secondary side of a steam generator and heating said solvent via heat transfer from the primary side of the plant (nuclear core decay heat and primary side recirculation pump heat) to the secondary side in “Mode 5.” Mode 5 is an industry and regulatory definition describing one of six operating modes ranging from power operations (Mode 1) to shutdown and “defueled” conditions (Mode 6). Mode 5 is a condition of plant operations during which no electric power is being produced by the plant (the reactor is subcritical), but fuel remains in the core, with the primary temperature initially from 210 to 200° F. (99 to 93° C.) cooling down to less than 100° F. (38° C.).
The cleaning process disclosed by Remark is said to last for a period described as 24 to 36 hours. Typically, the PWR plant would not stop cooling the plant during a shutdown to hold the temperature at the required cleaning temperature of 200 to 210° F. (99 to 93° C.). As such, the 24 to 36 hours represents what is known as “critical path” time, or time during which electricity is not being produced. The value of electric power produced for 24 to 36 hours can be more than US$1,000,000. It is also not clear that the 24 to 36 hours includes time to inject the cleaning chemicals and partially drain the steam generators for sampling. Several of the references cited herein would suggest that 24 to 36 hours of cleaning time may be inadequate at the temperatures cited in Remark, so actual critical path impact may be greater.
The Remark specification further describes the use of nitrogen sparging at 250 to 1500 cubic feet/minute (cfm) (7.1 to 42.5 m3/min) to promote mixing. The benefits of gas sparging for mixing of the fluid on the secondary side of a steam generator were studied in the 1980's (see, for example, EPRI-NP 2993 entitled “Evaluation of Steam Generator Fluid Mixing during Layup”). In this work, modeling and testing demonstrated that complete turnover of the liquid on the secondary side of an RSG could be achieved at flows from 10 to 30 cfm (0.28 to 0.85 m3/min) in as little as seven minutes. The mixing time was found to predicted by Equation 1 as provided below:Tmix=0.6 Q−5  (1)Where Tmix was the mixing time in hours, and Q was the gas flow rate in cfm. A 30 cfm (0.85 m3/min) flow corresponds to a 6 minute mixing time, typically more than adequate for most chemical cleaning operations.
The rates disclosed in Remark (250 to 1500 cfm) (7.1 to 42.5 m3/min) will undoubtedly promote mixing, but have the potential disadvantage of rapidly pressurizing the steam generator if a continuous vent path is not provided. The free space above the chemical cleaning solution during cleaning is on the order of 3000 to 4000 cubic feet in most RSGs. Therefore, depressurization may be required every few minutes at a gas flow rate of 1500 cfm (42.5 m3/min). Depressurization would only be required every few hours at 30 cfm (0.85 m3/min). Finally, high sparge rates also increase environmental emissions of volatile species such as ammonia (and other amines) and hydrazine, often present in chemical cleaning solutions.
The ability to promote mixing at low gas flows is also supported by other references such as Shah et al., “Flow Regimes in Bubble Columns,” AIChE Journal, 28 (182), pp. 353-379, and specifically for spargers such as those used in chemical cleanings or during sparging through the blowdown pipe, as discussed in Tilton, et al., “Designing Gas-Sparged Vessels for Mass Transfer,” Chemical Engineering, (November 1982).
Mixing of OTSGs with gas during chemical cleaning is also described in Buford (previously cited) through use of gas eductors.
A claimed advantage of the on-line process described in Remark is that it does not require that the steam generator be drained to install connections to the steam generators for the introduction, recirculation or draining of cleaning solvents. As described in Remark, off-line chemical cleaning processes usually require heating and cooling in a sequence of steps using external equipment set-up at a significant distance, up to 1500 feet (460 m) or more, from the SGs outside of the “containment building” which houses the steam generators. The distance is mandated by the need for a large “lay down” or set-up area for the external process heat equipment, and such space (typically more than 100,000 square feet) is generally not available directly adjacent to the containment building.
As described in Partridge and Gorman, numerous fluid and gas connections are made to the SGs in external heat processes. Each of these in turn requires a hose or piping to connect to the external chemical cleaning system. The external cleaning system includes a complex array of heaters, pumps, valves, storage tanks, coolers and controls. Inside of containment, there can also be literally hundreds of feet of piping, numerous pumps, and hundreds of valves. The time to set up the external process system even before the plant shutdown (after which interconnections to the steam generators are made) can range from one to three months. The time required to connect the external process system to the SGs can be an additional three to six days or more and involves up to four to twelve or more temporary adapters to be affixed to conventional access penetrations on the secondary side of the SGs. Once set up is complete, an external heat cleaning process typically requires from 5 to 10 days (144 to 240 hours) for each group or set of steam generators that are cleaned.
These adapters include supply and return lines for solvents and rinses, drains, level control instrumentation taps, pressure instrumentation taps, temperature indicator taps, gas sparging, corrosion monitoring electronics penetrations, and sample line taps. The necessity of many of these interconnections is to support external heating. The actual application time for the chemical cleaning ranges from several days to several weeks, depending on the complexity of the process (number of solvent steps, rinses, etc.). Demobilization including removing the temporary adapters from the steam generators requires several more days. Whether or not the set-up, application, and demobilization are on “critical path” depends on other plant refueling and maintenance activities that are underway. In many cases, particularly in longer refueling outages, external heat chemical cleaning processes have not affected critical path.
If heating is supplied from the primary side, as described in Remark and Kuhnke, the number of interconnections can be limited or eliminated. If no interconnections are made, other means for obtaining liquid samples and performing corrosion monitoring may be required, and these may be very difficult to implement or qualify (i.e., ensure the structural integrity and safe operation). The benefits of in situ corrosion monitoring during off-line processes (electronic corrosion monitors and coupons placed inside the SGs) is well established as reported in NP-2976 and in EPRI NP-5267 “Weld Region Corrosion During Chemical Cleaning of PWR Steam Generators” (July 1987). This is because essentially all chemical cleaning solvents will slightly corrode steam generator components including the pressure boundary shell and internal structures if fabricated from carbon and low allows steels. Typical corrosion allowances for these structures and components range from less than 0.001 to 0.010 inches (25.4 to 254 μm) for each cleaning application.
When installed inside a steam generator during a cleaning application, an in situ electrochemical corrosion monitoring system (CMS) allows for the nearly instantaneous detection of off-normal chemistry or process conditions that can lead to unacceptable corrosion. The importance of real-time corrosion monitoring is further supported by recent experience discussed in Dijoux, et al. In this reference, corrosion in some locations of one steam generator during an on-line chemical cleaning with no real-time electrochemical corrosion monitoring was reported to be 0.050 inches (1.27 mm) or five (5) times a typical corrosion allowance. The event was attributed to abnormal application conditions. The process did not use an in situ electrochemical CMS system which is considered the state-of-the-art method for corrosion monitoring during chemical cleaning. A CMS uses techniques including linear polarization resistance (LPR) and zero resistance ammetry (ZRA).
Sampling and analysis of the chemical cleaning solution as frequently as every 30 minutes is also critical to ensuring the process is proceeding as expected. Every chemical cleaning of a nuclear steam generator has included very strict requirements on chemistry of the solvents (see EPRI references cited above). As described in Partridge and Gorman, these samples can be taken from the recirculation loop or directly from sample lines on the temporary steam generator adapters during external cleaning processes. Because there is no external recirculation loop and no temporary penetrations into the steam generator during on-line/plant heat processes, partial draining of the steam generators is often required in order to sample cleaning solvents.
Based on the above, the primary advantage of on-line/plant heat processes for cleaning nuclear steam generators such as the method described in Remark is that this type of process requires a less complicated and labor-intensive equipment setup. On-line processes may also result in reduced schedule impact, although the actual impact to critical path schedule would be plant-specific (many off-line external heat chemical cleanings of nuclear steam generators have not impacted critical path). The primary disadvantage of on-line/plant heat processes is that process and corrosion monitoring may not be feasible or may be significantly more complicated, such that there is an increased potential for excessive corrosion, increased environmental impact, or other unwanted side effects. By comparison, traditional external cleaning processes are very safe in that they allow industry standard process monitoring techniques to be easily performed. However, typical equipment configurations used during external processes are complex, and require significant time and manpower to setup and operate.
A feature of the cleaning method using direct steam injection disclosed herein is that this type of process combines the advantages of on-line/plant heat and off-line/external heat processes, offering a method of external heating that results in a greatly simplified equipment setup, while at the same time allowing process monitoring equipment to be installed inside the steam generators during the cleaning. The specific advantages of the direct steam injection cleaning method, relative to traditional cleaning methods, include: (1) greatly simplified equipment configuration, including a simple method of external heating, (2) shorter set-up times and reduced manpower requirements, (3) shorter demobilization times, (4) steam generator access prior to the cleaning to facilitate installation of online corrosion monitoring equipment and coupons inside the steam generators, and (5) ability to perform liquid sampling without needing to partially drain the steam generator as described in Remark.
Previously, direct steam injection has not been used as a means for heating during cleaning of nuclear steam generators and related applications due to concerns that direct steam injection could lead to damage of vessel internals as a result of large thermal gradients or cavitation induced in the vicinity of steam injection equipment and/or vibration of steam injection equipment inside the vessel being cleaned. The direct steam injection method and apparatus disclosed herein have addressed these concerns and provide a means for introducing steam directly into nuclear steam generators or other vessels during cleaning applications with low thermal gradients in the vicinity of steam injection (e.g., below acceptable thermal gradients defined in design basis documents for nuclear steam generators or other heat exchanger equipment), and with minimal cavitation or vibration induced by steam flow, thereby preventing mechanical damage to vessel internals.
The method of cleaning with direct steam injection is applicable to conventional chemical cleaning processes as described in Frenier and the EPRI/SGOG references, as well as cleaning options such as those described in Rootham I, Rootham II and Varrin. The latter two patents describe uses of advanced “scale conditioning agents.” The method described herein may also be used with dispersant or decontamination solutions, or any other processes for cleaning heat exchangers or similar vessels, or removing waste such as nuclear waste from similar vessels or fluid systems where temperature control is required or helpful.